In its most primitive form “sandblasting” can be accomplished by simply gravity feeding an abrasive media such as sand from a hopper with a hole in the bottom into a pressurized air stream. A slightly more sophisticated version of this is to use a pressurized powder tank instead of a hopper. This combination of pressure and gravity allows for a more consistent flow of the media. A simple metering orifice at the nozzle tip also helps with consistent delivery. These crude forms of abrasive blasting may be adequate for removing rust from an old car body prior to repainting, but they leave much to be desired especially when macro and micro abrading tasks are the norm in today's high technology fields. So the industry's goal has been to develop more sophisticated devices that feature precision, repeatable and consistent control of a fine air abrasion stream. However, fine abrasive powders and surface conditioning media in the 10 to 100 micron range used in these systems do not like to flow easily or consistently! Getting them out of a storage chamber into an air stream in a precision, consistent volume has been problematic. There are numerous reasons for this. Due to their bulk density and or cohesive nature, fine particles like to agglomerate and can result in phenomena such as: caking, bridging or “rat holing” when the media is housed in a hopper or chamber. Prior Art has provided various solutions to these flow problems, some being more successful than others, but each having disadvantages as well.
Some early systems utilized pressurized vessels with incoming air forced through a nozzle internal to the tank with a tip configured to create an excitation or “sandstorm” of the abrasive particles within. This air/particle mix was then propelled out through a port to a nozzle tip. Flow rate for this approach is dependent on how much abrasive is in the hopper. Results are sporadic and lacks repeatability and are further complicated when finer particles, (below 50 micron) are required.
Another step forward in the quest for more consistent powder feed was a chamber configured like a funnel so that gravity would influence abrasive particles to feed through an orifice in the bottom of the chamber and out through a cross hole to the nozzle tip. A further improvement was the addition of a pulsing action on the input side of the pressure tank which encouraged the powder to flow out to the nozzle tip. This scheme offers a refinement but the powder flow is not independent of air pressure and not adjustable. Also as the powder hopper goes from full towards empty the flow rate varies. Furthermore, when changing over from one abrasive type to another, it may be necessary to change the internal orifice in the bottom of the powder chamber a task that can take several minutes.
A significant improvement was the vibratory powder feed system. The first of these consisted of a powder tank mounted on a vibrator. With an assist from gravity, powder was vibrated down through a multi hole orifice plate into a mixing chamber where incoming air would pick up the abrasive particles and deliver them through the exit port and out to the nozzle tip. Abrasive powder flow is adjusted by turning the amplitude of the vibrator via a rheostat. See Black, U.S. Pat. No. 2,696,049.
Although a big step forward in powder flow flexibility, repeatability and consistency the downside for cycles of a short duration (stop and go) is a puff of abrasive at the start of each cycle caused by a venturi like effect when a burst of powder has been sucked into the air stream. Also after the unit has been off for a time the first cycle presents a burst of powder, because due to gravity, abrasive has sifted down through the orifice plate.
A later vibratory feed system consisted of an upper hopper via gravity feeding a lower mixing chamber configured like a bowl feeder mounted on a vibrator coil mounted within an outer tank. The vibration of inner tank would induce the abrasive particles to spiral upward, on a track, where they would then escape though an exit orifice. See Gallant, Kulischenko, U.S. Pat. No. 4,733,503. While eliminating the gravity powder burst problem this design has an issue with abrasive cascading over the rim of the bowl feed chamber into the outer chamber and eventually damping down the vibrator coil within the outer pressure chamber. There is also an issue with the fact that in use the coil heats up and causes the flow rate to fluctuate. (This means the coil must be kept warm at all times). Furthermore the vibrator amplitude and hence the flow rate is not easily adjusted. This results in a very temperamental system.
Yet another type of system consisted of a funnel shaped pressurized hopper with powder vibrated by a ball and raceway mechanism, the ball orbiting around the perimeter of the exit orifice is propelled by pneumatic pressure. The ball bearing orbiting the orifice along with gravity induces the powder to flow out through an intersecting cross hole orifice in the bottom of the chamber. Up from the bottom of the orifice is a throttling needle like device, also described as a pintle. Pivoting upwards or downwards by moving a lever, the pintle creates an adjustable gap at the bottom of the chamber to allow for variable powder flow rates. See Shipman U.S. Pat. No. 4,569,161. While offering fairly consistent flow rates the downside of this approach is the fact that the pneumatic ball bearing race used to agitate the abrasive powder can tax a compressor as it uses considerable amount of air.
As described above each of these approaches work to a certain degree but each has its inherent weaknesses.